Unlike 2004 disaster, faults in crust moved from side to side, instead of up and down

Below:

Next story in Science

A graphic from the U.S. Geological Survey shows recent seismic activity in the Indian-Pacific Ocean region. The size of the boxes represents the magnitude of earthquakes. Orange boxes mark quakes that have occurred over the previous 24 hours, and yellow denotes quakes that have occurred during the previous week. The cluster of boxes off the coast of Sumatra shows the location of the magnitude-8.6 quake and its aftershocks.

The magnitude-8.6 earthquake that struck in the Indian Ocean off the western coast of Sumatra today resurrected fears of a repeat of the 2004 Indian Ocean earthquake and tsunami that proved one of the most devastating natural disasters in modern memory.

However, this earthquake, which struck at 2:38 p.m. local time (4:38 a.m. ET), about 270 miles (435 kilometers) off the coast of the Indonesian island, was a different animal altogether than the 2004 earthquake and tsunami, which killed more than 230,000 people and left millions homeless.

"It was quite a bit smaller," said Julie Dutton, a geophysicist with the U.S. Geological Survey. The 2004 quake was a magnitude-9.1 — the third most powerful earthquake ever recorded.

Perhaps more significantly, today's earthquake was a different kind of quake all together. Instead of occurring at a plate boundary along an area called a subduction zone, where one tectonic plate is diving beneath another, this earthquake occurred in the middle of an oceanic plate, where the faults in the crust essentially moved from side to side instead of up and down. These sorts of events are called strike-slip earthquakes.

"With a strike-slip event you don't have the same potential hazard for a tsunami as you do with a subduction event because the plates are moving adjacent to each other," Dutton told OurAmazingPlanet.

Although they are sometimes produced by landslides on the seafloor, significant tsunamis are typically created by subduction earthquakes, when one massive oceanic plate suddenly lurches deeper beneath another plate, shoving up a huge section of the seafloor. That displacement of the ocean floor also displaces ocean water. Essentially, the more ocean floor you move, and the more dramatically you move it, the more water you move, and the bigger the tsunami you get.

Just minutes after the earthquake struck today, the U.S. Pacific Tsunami Warning Center in Hawaii issued a tsunami watch for the entire Indian Ocean region.

A little more than an hour after the first and largest quake hit, the watch was still in effect, and tsunami wave heights of 1 foot were recorded in Sabang, in Indonesia's Banda Aceh province, the region that was most affected by the 2004 disaster.

Thousands of earthquakes happen every day around the world. Most are hardly felt, if at all. But sometimes pieces of Earth's crust suddenly slip past each other in a massive release of pent-up stress. The jolted Earth rumbles, buildings collapse, streets buckle, and thousands of people die. These movements are nature's most violent act and take a grim toll on human life and infrastructure.

The deadliest earthquake in recorded history rattled the Shensi province of China on Jan. 23, 1556, and killed an estimated 830,000 people. The death toll was particularly high among peasants who lived in artificial caves that were dug into soft rock and collapsed during the quake. This picture shows a pagoda whose peaked top was lost in the shaking. Earthquake damage is also visible on the corners. Click on the "Next" label to learn about 11 more deadly quakes.

— John Roach, msnbc.com contributor

1906: The Great Quake

USGS

The California earthquake of April 18, 1906, ranks as the most deadly in U.S. history: About 3,000 people perished. The Great Quake, as the event is known, was estimated at magnitude 7.9 and ruptured along 296 miles of the northernmost section of the San Andreas fault. Broken gas lines, fractured chimneys and toppled chemical trucks sparked a series of fires that torched large sections of San Francisco, as seen in this image taken from Golden Gate Park.

The most powerful earthquake in North American history shook the state of Alaska on March 27, 1964, the Friday before Easter. The magnitude-9.2 temblor triggered a tsunami that was responsible for 113 of the 128 deaths associated with the earthquake, according to the U.S. Geological Survey. The giant waves struck along the West Coast down to California, and rolled across the Pacific to Hawaii. This image shows the coastal town of Seward, Alaska, in the wake of the tsunami.

1970: Mountains moved

USGS

The magnitude-7.9 earthquake that struck just off the west coast of Peru on May 31, 1970, reduced the coastal towns of Casma and Chimbote to rubble and killed at least 3,000 people. Even greater disaster struck the towns of Yungay and Ranranhirca. The shaking sent an avalanche of mud, rock and ice down the slopes of the Cordillera Blanca and buried the cities under tens of feet of debris. An estimated 70,000 lives were lost. Here, a statue of Christ is all that remains in Yungay.

1976: Chinese region flattened

USGS

The deadliest earthquake in modern times flattened the industrial city of Tangshan, China, in the early morning of July 28, 1976. The Chinese government put the death toll at 255,000, though many geologists believe it was much higher — up to 655,000. Nearly 800,000 more were injured. Tremors and damage from the magnitude-7.5 quake extended as far as Beijing, about 90 miles from the epicenter. Here, a few tents and temporary shelters are visible amid the debris.

On Sept. 19, 1985, a magnitude-8.2 earthquake off Mexico's Pacific coast wreaked the greatest havoc in Mexico City, about 220 miles from the epicenter. There, hundreds of buildings were toppled, and thousands of people died. Government officials put the death toll at about 9,000, though other sources say it may have been as high as 35,000. A triggered tsunami sent waves rising almost 10 feet crashing into the coastal towns of Lazaro Cardenas, Zihuatanejo and Manzanillo. Here, a 21-story steel-constructed building in Mexico City lies in ruins.

1995: Tremors hit Japan

Roger Hutchison via NGDC/NOAA

More than 6,400 people died in the aftermath of a magnitude-6.8 earthquake that hit Japan on Jan. 17, 1995. Most of those deaths occurred in Kobe, the city closest to the epicenter. Many buildings suffered partial collapse, such as the one shown in this picture. Total damage was estimated at more than $100 billion.

2003: Iranian city crumbles

Majid
/
Getty Images

On Dec. 26, 2003, a magnitude-6.6 earthquake crumpled the adobe city of Bam, Iran, killing an estimated 30,000 people. About 60 percent of the city's buildings were destroyed and nearly all the rest were damaged. The event ranks as the deadliest in Iran's history. Here, one of the victims is carried to the grave.

On Dec. 26, 2004, a magnitude-9.1 earthquake ruptured the ocean floor off the west coast of Sumatra, Indonesia, and triggered a series of destructive tsunamis that killed at least 225,000 people in 11 countries. Millions more were stripped of their homes. Scientists estimate the energy released in the event was more than 1,500 times the power of the atomic bomb dropped on Hiroshima in 1945. Here, villagers walk through a devastated area of Pangdandaran on the Indonesian island of Java.

2005: Landslides in Kashmir

David Guttenfelder
/
AP file

At least 86,000 people were killed when a magnitude-7.6 earthquake hit the Kashmir region of northern Pakistan on Oct. 8, 2005. Millions more were left homeless at the outset of the harsh Himalayan winter. Landslides swept away villages and blocked roads for relief and rescue workers, worsening the human toll. At least 1,350 people were killed in neighboring India, and the shaking was also felt in Afghanistan. Here, rescue workers dig through the rubble looking for survivors at a school in Balakot, Pakistan.

2008: Catastrophe in China

Xinhua via AFP - Getty Images

An estimated 70,000 people died and millions were left homeless when a magnitude-7.9 earthquake hit a region north of Sichuan's provincial capital, Chengdu, on May 12, 2008. Tremors were felt as far away as Beijing and Shanghai. One of the most tragic episodes was the collapse of a high school in Juyuan. This picture shows searchers digging through the school's rubble.

Extreme poverty and extremely poor construction standards contributed to the devastation and death in Haiti when a magnitude-7.0 quake hit Port-au-Prince and its surroundings on Jan. 12, 2010. The death toll amounted to more than 230,000, and aid officials say it will take years for Haiti to recover fully ... if it ever can. This picture shows rescuers carrying one of the injured away from the rubble.